"Using a formula based on brain size, Dunbar estimated that the Macaca sinica (pictured below) tends to run in groups of around 17, while the Cacajao tends to have about four monkey pals. When Dunbar appliedhis formula to humans, he predicted that the typical social group size — that’s the largest number of individuals that we humans can maintain stable relationships with — would be 147.8 (to be precise, he estimated it would be somewhere between 100 and 231 people). To test his theory, Dunbar started off by looking at modern hunter-gatherer societies, where he found three levels of social cohesion. On one end was the small living groups or overnight camps (which had between 30 and 50 people); on the other was the large population unit or tribe (which had between 500 and 2,500 individuals). Between those two levels was the clan, which typically contained between 100 and 200 people. That’s darn close to Dunbar’s estimate of the number of relationships people can keep track of. [..] When Dunbar looked at the smallest independent unit in various armies, he found that the average size was 179.6 — again, within the bounds of his original estimate of social group size. And in a follow-up study he wrote in 2002, Dunbar found that Christmas cards were on average sent out to 153.5 individuals."